Neoproterozoic to Lower Paleozoic Sequences of the Congo Shield: Comparisons Between the Congo and Its Peripheral Basins
E´tienne Kadima Kabongo, Damien Delvaux, Michel Everaerts, Mwene Ntabwoba Stanislas Sebagenzi, and Francis Lucazeau
The Congo Basin (CB) is the largest and the least explored African continental sedimentary basin. Outcrop scarcity, intense weathering and dense forest constitute obstacles to undertake direct geologically-based exploration programmes, highlighting the importance of airborne and ground-based geophysics. In the 1950s, the ‘ Syndicat pour l’e´tude ge´ologique et minie`re de la Cuvette congolaise ’ conducted refraction and reflection seismic, gravimetric and magnetic field surveys across the central parts of the CB (Evrard 1957). During the seismic surveys, 117 refrac- tion stations were employed in the basin interior, and 7 reflection lines were recorded along rivers (Evrard 1960).
In addition, about 6,000 gravity and magnetic measurements were made with an average station spacing of 5 km along rivers and dirt roads (Jones et al. 1960). Moreover, two fully- cored stratigraphic wells down to ca. 2,000 m had been drilled in the 1950s, reaching red sandstones at the bottom, that were interpreted as pre-Carboniferous (Cahen et al. 1959, 1960).
Using these geophysical and well data, Evrard (1957, 1960) and Jones et al. (1960) first imaged the structure and extent of the CB and its subsurface geometry. The major refractors mapped provided the first regional overview of the
basin subsurface structure. The CB appeared deeper than previously thought, with the possible existence of several thousand meters of sedimentary rocks beneath its upper Paleozoic–Mesozoic sedimentary sequences.
Additional geophysical surveys were subsequently conducted by various petroleum companies. Exxon-Texaco shot 2,900 km of seismic reflection profiles in 1974–1976, and Japan Oil National Corporation (JNOC) performed an aeromagnetic survey and gravity measurements around Kisangani in 1984; and two additional wells, more than 4,000 m deep, were drilled (Esso Zaire 1981a, b). The interpretation of this data was first presented in an ECL (1988) report by Petrozaı¨re, of which a short summary was published by Lawrence and Makazu (1988). More detailed interpretations of the seismic reflection profiles and wells were presented as synthetic stratigraphic columns and a tectonic model by Daly et al. (1992), in which early subsi- dence of the basin was interpreted to reflect Neoproterozoic processes of rifting and thermal relaxation, followed by two regional contractional deformation phases that inverted the subsiding basin during the early Paleozoic and the late Paleozoic. The first inversion was attributed to late stages of the Pan-African orogeny in Central Africa, and the second inversion phase to far field intraplate stresses generated at the southern margin of Gondwana during the formation of the Cape Fold Belt. Daly et al. (1992) also speculated on the possible role of evaporites in enhancing deformation process at the deepest levels of the basin. Subsequently, Kadima et al. (2011a) further constrained the interpretation of some of the seismic profiles using 2D gravity and magnetic models, and delineated heterogeneous crust beneath the sedimentary sequences of the basin consistent with the inter- pretation that the central part of the basin is an intracratonic rift inverted during compressional tectonics facilitated by the presence of evaporites and salt tectonics.
Despite a renewed interest over the last decade, there are still many unanswered questions regarding the nature of the crystalline basement, the Neoproterozoic and Phanerozoic stratigraphy, as well as the structure of the CB. The E ´ .K. Kabongo ( * ) M.N.S. Sebagenzi
Laboratoire de Ge´ophysique et Ge´odynamique, De´partement de Ge´ologie, Universite´ de Lubumbashi, Lubumbashi, DR Congo e-mail: email@example.com; firstname.lastname@example.org D. Delvaux M. Everaerts
Geodynamics and Mineral Resources, Royal Museum for Central Africa, 3080 Tervuren, Belgium
email@example.com F. Lucazeau
Dynamique des Fluides Ge´ologiques, Institut de Physique du Globe de Paris/Sorbonne, Paris Cite´, UMR CNRS 7154, 1 rue Jussieu, 75005 Paris, France
M.J. de Wit et al. (eds.), Geology and Resource Potential of the Congo Basin, Regional Geology Reviews, DOI 10.1007/978-3-642-29482-2_6, # Springer-Verlag Berlin Heidelberg 2015
crystalline basement comprises Archean to Mesoproterozoic igneous and metamorphic rocks of the Congo Shield (see de Wit and Linol, Chap. 2, this Book). Scarce outcrops, limited geological data and stratigraphic age constraints still prevent a detailed reconstruction of the Neoproterozoic history of the CB. Nevertheless, many authors have attempted to decipher the Neoproterozoic geological evolution of Central Africa and the CB in particular (Cahen 1954; Evrard 1960;
Lawrence and Makazu 1988; Daly et al 1991, 1992; Kadima 2011; Kadima et al. 2011a, b; Linol 2013; see also Chaps.
2 to 5, this Book).
Here, we re-examine the largely hidden Neoproterozoic to lower Paleozoic sequences of the CB near the centre of the Congo Shield and compare its evolution with the surrounding and partially exposed marginal sedimentary basins.
6.2 Neoproterozoic to Lower Paleozoic Sequences of the Congo Shield Flanking the CB
In Central Africa, the Neoproterozoic geological evolution is marked by a succession of major events initiated by the break-up of the Rodinia supercontinent and opening of rift- basins (e.g. West Congolian, Sembe´-Ouesso, Mbuji-Mayi, Katanga, Zambian basins), and terminating by the amalgam- ation of Gondwana and the filling of associated foreland basins (e.g. de Waele et al. 2008). Traces of these events can be observed within the Congo Shield (sensu Stankiewicz and de Witt 2013) and particularly in marginal sedimentary basins surrounding the CB (Fig. 6.1a), such as the West Congo, Sembe´-Ouesso, Bangui and Liki-Bembian basins to the West and Northwest; the Fouroumbala-Bakouma and Lindian basins to the North and Northeast; the Itombwe and Malagarazi-Bukoban basins to the East; the Katanga, Zambian and Mbuji-Mayi basins to the Southeast (Fig. 6.1b).
Studies based on shear-wave tomography (Crosby et al.
2010; Priestley et al. 2008; Ritsema and van Heijst 2000), admittance models (Downey and Gurnis 2009; Hartley and Allen 1994; Hartley et al. 1996; Pe´rez-Gussinye´ et al. 2009), kimberlite data (Crosby et al. 2010; Batumike et al. 2009), subsidence modelling (Kadima et al. 2011b; Kadima 2011) and heat flow estimations (Kadima et al. 2011b; see also Lucazeau et al., Chap. 12, this Book) suggest that the CB and surrounding Lindian and Mbuji-Mayi basins are underlain by a thick mantle lithosphere (200 km or more), with an equivalent elastic thickness of ca. 100 km (for further details see Raveloson et al., Chap. 1, this Book).
Evidence of Rodinia break-up as main mechanism of formation for some of the sedimentary basins surrounding the CB is supported by the age of magmatic intrusions
observed into these basins (Vicat and Vellutini 1987; Vicat et al. 1989; Key et al. 2001; Tack et al. 2001; Johnson et al.
2007; de Waele et al. 2008). A brief summary of the strati- graphy of these marginal basins is given below, in clockwise order as depicted in Fig. 6.1:
1. The West Congo basin contains rhyolites and felsic volcanoclastics dated at 950 Ma (Tack et al. 2001) and intruded by granitoids (e.g. in the West Congo Super- group and the Sangha-Comba Aulacogen). These volcano-magmatic series have been considered to repre- sent a continental rift sequence related to the break-up of Rodinia and the opening of the Adamastor Ocean with the deposition of passive margin sediments (Alvarez 1995a, b; Tack et al. 2001). These sequences are overlain by the Ediacaran-Cryogenian West Congo Group comprising a succession of siliciclastics interbedded with diamictites and carbonates (Frimmel et al. 2006; see also Delpomdor et al., Chap. 3, this Book).
2. The Sembe´-Ouesso basin exposed in the Republic of the Congo (Congo-Brazzaville) contains sequence of schist and quartzite, overlain by a pelitic and sandstone sequence, and in turn by the Dja tillite, estimated at ca.
950 Ma (Vicat and Vellutini 1987). Dolerites intrude all these sequences; and their emplacement is interpreted to be related to crustal extension leading to basin subsidence in a failed rift (Vicat and Vellutini 1987; Vicat et al. 1989;
Vicat and Pouclet 1995).
3. The Fouroumbala-Bakouma and Bangui basins in the Central African Republic (CAR), the Liki-Bembian, Lindian, Itombwe syncline in the Democratic Republic of Congo (DRC), as well as the Malagarazi-Bukoban basin in Burundi and Tanzania all have been recognized to be Neoproterozoic in age, based on limited radiometric dating and lithostratigraphic correlations (Alvarez 1995a, b, 1999; Poidevin et al. 1980/1981; Poidevin 1985, 2007;
Deblond et al. 2001). The age of the Bakouma Formation was first estimated at ca. 840 50 Ma (Poidevin 1996), although Alvarez (1999) obtained a whole rock Rb-Sr date for these rocks of ca. 683 11 Ma. Magmatic intrusions have been documented, in particular in the Fouroumbala-Bakouma, Bangui, Itombwe and Malagarazi-Bukoban basins (Waleffe 1988; Poidevin 1976, 1979; De Paepe et al. 1991; Tack 1995; Deblond et al. 2001). Amygdaloid lava belonging to the Malagarazi Supergroup has been dated at 795 7 Ma (Deblond et al. 2001). Syenite, dated at 700 50 Ma, intrude sediments filling the Itombwe basin (Villeneuve 1983).
4. The Katangan and Zambian basins, now part of the Lufilian (Arc) fold-and-thrust belt, developed during two magmatic events between 880 Ma and 750 Ma (Porada 1989; Armstrong et al. 2005; Johnson et al.
2005), and were subsequently deformed during the Pan-
African orogen, between ca. 700 Ma and ca. 530 Ma (Cailteux 1994; Hanson et al. 1994; Porada and Berhorst 2000; Key et al. 2001; De Waele et al. 2008). These basins comprise the Roan, Mwashya, Nguba and Kundelungu Groups. The Roan Group overlies the 877 11 Ma Nchanga granite (Armstrong et al. 2005), and the Mwashya Group contains 765 5 Ma intrusions (Key et al. 2001).
5. The Mbuji-Mayi basin is a SE–NW trending intracratonic failed-rift basin. It contains a lower clastic sequence BI and an upper carbonate sequence BII (Raucq 1957, 1970;
Cahen et al. 1984; Delpomdor, et al. 2013a). The pres- ence of dolerite intrusions and pillow lavas were taken as evidence of extensional magmatism during the deposition of the Mbuji-Mayi sediments (Raucq 1957, 1970; Cahen et al. 1984). Isotopic data from the Mbuji-Mayi carbo- nates and the presence of pseudomorphs of anhydrite and gypsum filling veins and fractures reflect deposition and early diagenesis in marine and evaporitic conditions (Delpomdor, et al. 2013b; see also Delpomdor et al., Chap. 4, this Book). The minimum age of sedimentation for the Mbuji-Mayi Supergroup was first estimated by amygdaloid basalts interpreted as capping the entire sequence and dated at ca. 940 Ma (Cahen et al. 1984).
Delpomdor et al. (2013a) further constrained the deposi- tion of this Group between 1174 22 Ma and ca.
800 Ma; and the carbonate BII Group is now dated at 760–820 Ma by Carbon and Strontium chemical stratig- raphy (Delpomdor et al., Chap. 4, this Book) and the sliciclastic BI Group is either older than 880 Ma or aged between 880 Ma and 850 Ma. Following this second hypothesis, the Mbuji-Mayi Supergroup would be coeval with the Roan Group in the Katanga and Zambian basins, reflecting a similar early Neoproterozoic exten- sion event.
6.2.1 Neoproterozoic Siliciclastic and Carbonate Sequences
The Neoproterozoic basins surrounding the CB have consis- tent (similar) stratigraphic successions. In most of them, a carbonate unit, of variable thickness, overlies a basal silici- clastic sequence comprising silts, sandstones and conglo- merates. The Neoproterozoic carbonates are in turn overlain by relatively thick and persistent siliciclastic sequences.
For example, in the West-Congo basin, the Schisto- Calcaire Subgroup overlies the Haut Shiloango Subgroup and is overlain by the Mpioka Subgroup. The Haut Shiloango Subgroup comprises predominantly carbonates, with increasing siliciclastics upwards (Delpomdor et al.
2014; and Chap. 4, this Book). The Mpioka Subgroup consists of up to 1,000 m of molasse-like sequences com- prising conglomerates, sandstones and argillites (Alvarez 1995a, b; Tack et al. 2001; Frimmel et al. 2006). Similarly, in the Bangui basin, the basal Kembe sequence of conglomerates is overlain by the Bimbo sandstones and the Bangui carbonates. Here, no clastic sequence is observed above this carbonate sequence (Poidevin 1976).
A similar succession is observed in the Fouroumbala- Bakouma basin (Alvarez 1995a, b; Tait et al. 2011). In the Lindian basin, the Ituri Group consists of a basal clastic sequence containing conglomerates and sandstones, overlain by thick oolitic limestones and dolomites. Above it, the Lokoma and Aruwimi Groups are predominantly clastic (Verbeek 1970; Poidevin 2007). In both the Katangan and Zambian basins, the Roan Group (880–750 Ma) consists of a basal conglomerate, siliciclastics and carbonates (mainly dolomites and dolomitic shales) that unconformably overly Mesoproterozoic Kibaran formations (Cailteux et al. 2005;
Batumike et al. 2006; El Desouky et al. 2008). The Roan Group is overlain by the predominantly clastic and volcano- sedimentary rocks of the Mwashya the Nguba Groups (750–620 Ma). The latter contain a basal tillite known as the “ Grand Conglomerat ” (Lepersonne 1974), overlain by a thick carbonate sequence including limestones, dolomites, shales, dolomitic shales and interbbeded silts and sandstones (Cailteux et al. 2005; Batumike et al. 2007). The overlying Kundelungu molasses-like sequence (620–570 Ma) starts with a second glacial horizon, known as “ Petit Conglomerat ” (Lepersonne 1974).
6.2.2 RedBeds and the Transition Between the Neoproterozoic and Paleozoic
Throughout Central Africa, the transition between the upper Neoproterozoic and the earliest Paleozoic sedimentary sequences is poorly constrained because the transition sequences are non-fossiliferous siliciclastic redbeds.
Throughout the CB and its surrounding basins, relatively thick sequences of lithologically similar red sandstones (known as the ‘ Redbeds ’ ) have long been correlated and
Fig. 6.1 (Continued) (a) Geological setting of the Congo Shield with the Congo Basin in its center, surrounded by Archean cratonic blocks and Proterozoic mobile belts. Rectangle shows contour of (b). (b) Geo- logical map of the Congo basin and surrounding marginal
Neoproterozoic basins with location of the 2 stratigraphic wells
(Samba and Dekese) and the two exploration wells (Mbandaka-1 and
Dekese-1), indicated by their initials
assigned to the late Precambrian (Cahen et al. 1960; Verbeek 1970; Lepersonne 1974), although some researchers recognized parts could also be early Paleozoic in age (Evrard 1957). Subsequently many authors have tried to define the Neoproterozoic-Paleozoic transition in the basins surrounding the CB on the basis rather of structural criteria in Katanga and West Congo; and on the basis of lithological correlations with the Lindian Supergroup (Kampunzu and Cailteux 1999; Porada and Berhorst 2000; Master et al.
2005; Tack et al. 2008; Tait et al. 2011).
In West-Congo and Katanga, where these Redbeds occur along the tectonic fronts of the West-Congo and Lufilian fold belts, and from there extend towards the CB into their foreland, structural relations with reference to a supra- regional late Pan-African unconformity have been used to distinguish the red sandstones as both pre- and post-dating Pan-African deformation. In the West-Congo belt, for exam- ple, the Pan-African deformation has been dated at 566 42 Ma, the age of metamorphism of a dolerite sill that intrudes the Haut Shiloango Subgroup and the Inkisi Subgroup redbeds (Alvarez 1999; Tack et al. 2001; Frimmel et al. 2006). The Inkisi Subgroup is therefore considered as of latest Neoproterozoic to early Paleozoic age (Alvarez et al. 1995c; Tack et al. 2008; Tait et al. 2011). Recently, Monie´ et al. (2012) determined that the West Congo belt of Angola underwent two main deformation events of amphib- olite grade at c. 540 and 490 Ma, followed by tectonically assisted exhumation.
Similarly, in the Katanga basin, the sub-horizontal siliciclastic sequences of the Biano (Plateaux) Subgroup (uppermost subgroup of the Kudelungu Group) discordantly overlie the folded Katanga units and are considered therefore to be post Pan-African in age (Batumike et al. 2007;
Kampunzu and Cailteux 1999). The Lufilian Arc formed during collision between the Congo and Kalahari Shields between 650 and 530 Ma, with deformation peaking at ca.
550 Ma (Porada 1989; Hanson et al. 1993; Kampunzu and Cailteux 1999; Porada and Berhorst 2000; John et al. 2004;
Frimmel et al. 2006). Detrital muscovites from the Biano Subgroup have a maximum Ar/Ar age of 573 5 Ma at the top of the Katanga Supergroup (Master et al. 2005), while the minimum age of the Biano Subgroup is estimated as younger than 540 Ma (Kampunzu and Cailteux 1999). This suggests that at least some of the Biano sequences may have been deposited during the Cambrian (Kipata 2013).
In the sub-horizontal intracratonic Lindian Supergroup along the northern margin of the CB, no clear angular dis- cordance has been observed within the Aruwimi Group, but the Banalia red sandstones that form the upper part of this group, have been correlated lithostratigraphically with the Inkisi Group in West-Congo and the Biano (Plateaux) Sub- group in Katanga (Alvarez et al. 1995; Tack et al. 2008; Tait et al. 2011), but there are no direct age constraints.
Clearly throughout the Congo Shield, robust correlations between RedBeds sequences require more precise chronostratigraphy.
6.3 Structure and Stratigraphy of the Deep CB Based on Geophysics and Well Data
Since most of the CB is not accessible to direct observations, geophysical investigations can complement the surface geo- logy observations described above. Here we summarise Fig. 6.2 (a) Seismic-stratigraphy of sequence on a section of seismic profile R5, showing six depositional sequences, two unconformities and the calibrated trace of Mbandaka-1 well as identified in this chapter.
TWT: Two Way Travel-time (TWT in seconds). Vertical black line:
position of the well, red crosses: base of the seismic sequences
(corresponding depth in Table 6.2a). CDP 2231: Common Depth
Point location. (b) Time–Depth curve obtained from the CDP location
2231 along line R05, near the Mbandaka-1 projected trace well. The
curve shows the bottom limit of the different seismic sequences and the
well TD (4,350 m). Crosses: CDP data from Table 6.1a. The
depth–time curve was constructed by a polynomial interpolation of
the CDP data. Empty squares: depth–time plot of the base of the
seismic sequences from data of Table 6.2a
interpretations of the seismic and gravity data to further unravel the early history of the CB.
Whilst a distinct seismic reflector recognized below the CB has been interpreted to record Pan-African orogenesis (Daly et al. 1991, 1992; Kadima et al. 2011a), it only locally resolves a tectonic unconformity (for example on lines L50 and L51 in the centre of the basin; Figs. 6.3 and 6.4: marked as ‘ Lower unconformity ’ ). More often the reflector marks a conformable contact (see for example, line R5 calibrated against the Mbandaka-1 well; Fig. 6.2a).
6.3.1 Well Data
The original lithostratigraphic scheme of the central CB was established from logging the two fully cored stratigraphic wells: Samba and Dekese (Cahen et al 1959, 1960) and the two exploration wells: Mbandaka-1 and Gilson-1 (Esso Zaire 1981a, b) (location on Fig. 6.1). The Jurassic to Creta- ceous sequences that form the uppermost 700–1,200 m are relatively well described and dated by biostratigraphy (Colin 1994; Linol 2013; Linol et al., Chap. 8, this Book). Below, the Permo-Carboniferous sequences of the Lukuga Group were recognized in the Dekese well on the basis of their fossil plants and spores, and their glacial to peri-glacial characteristics. These were not encountered in the Samba well but suspected in the Mbandaka-1 and Gilson-1 wells. In the latter two deeper wells, these sequences overlie sili- clastics with dolomites (some with stromatolites) assigned to the Neoproterozoic, although the transition between the Precambrian and Paleozoic is poorly defined (see detailed lithostratigraphic subdivision of the four wells in Linol et al., Chap. 7, this Book). The Mbandaka-1 borehole stops at the depth of 4,350 m in a soft basement, interpreted to be halite (Esso Zaire 1981a).
6.3.2 Velocity Structure Based on Seismic Refraction Data
Evrard (1957) defined for the first time the P-wave velocity structure of the CB, based on the results of a refraction seismic survey calibrated with surface and well data at Samba and Dekese. Whilst Evrard (1957) stressed that this velocity structure is based on the physical characteristics of the sedimentary rocks and does not directly reflect the strati- graphy, the following synthesis seems robust: P-wave velo- cities < 3,600 m s–1
characterize the Mesozoic-Cenozoic sequences whilst velocities of ca. 3,900 m s–1
are typical for the middle to upper Paleozoic sequences. Higher velocities, between 4,200 and 4,600 m s–1
, likely represent the late Neoproterozoic to lower Paleozoic RedBeds, whilst
velocities > 5,000 m s–1
should represent the Neoprotero- zoic or older sediments and possibly crystalline basement.
6.3.3 Seismic-Stratigraphic Sequences
Using the seismic profiles acquired by Exxon-Texaco in 1974–1976 (location on Fig. 6.1), ECL (1988) and Daly et al. (1992) distinguished six seismic-stratigraphic sequences (or “ Supersequences ” ) above the acoustic crystal- line basement, each bounded by regional unconformities.
These sequences are re-described below (using the same numbering as in Daly et al. 1992) with reference to the seismic profile R5 shot along the Congo River and passing close to the Mbandaka-1 well (Figs. 6.1b, 2a). They are also illustrated on the land seismic profile L51 whose northern extremity approaches the Samba well (Figs. 6.1b, 3).
On the R5 profile (Fig. 6.2a), the depth-time curve is presented for CDP (Common Depth Point) location 2,231, measured closest to the Mbandaka-1 well (Fig. 6.2b). The depth-time curve was constructed by a polynomial interpo- lation of the CDP data (Table 6.1a). This allowed calculating the depth of the base of the different seismic-stratigraphic sequences and their thicknesses, as identified on the seismic line R5 (Table 6.2a). For reference, the bottom (TD) of the Mbandaka-1 well at 4,350 m deep correspond to a TWT time of 2,139 ms. Similarly, the depth to the base of the sequences identified on Line L51 have been calibrated for CDP 850 (Fig. 6.3, Tables 6.1a and 6.2b).
Except for the uppermost sequence (5), the ages of all sequences is poorly defined. Two major unconformities are identified and traced in most of the seismic profiles Fig. 6.3 Seismic stratigraphy of sequences identified on a section of profile L51 in the CB and linked further north to the Samba well (Fig. 6.1b) and two major unconformities as interpreted in this chapter.
The first four sequences beneath the lower unconformity are deformed
on the left (SW) side of the profile. TWT Two Way Travel-time ( in
seconds). Vertical black line: position of CDP 850, red crosses: base of
the seismic sequences (corresponding depth in Table 6.2b)
(relatively distant one from another). However their age control is also poor (especially for the lower one) and it is possible that there are more than 2 regional unconformities (e.g. Daly et al. 1992 identified 6 unconformities).
184.108.40.206 Sequence 0
Along several profiles, a reflection-free seismic pattern that corresponds to the acoustic basement is interpreted to represents crystalline basement. The latter is overlain by a low-amplitude, discontinuous and transparent seismic patterns forming the first sequence (Sequence 0 of Daly et al. 1992). The reflectors diverge towards the centre of the CB, suggesting tilting during deposition; and the sequence is thicker in the centre of the basin than at its border. Sequence 0 is apparently discontinuous, with rapid variations in thickness and its base generally is not well- imaged. Sequence 0 was not penetrated by any cored wells.
It could represent siliciclastic sediments (essentially con- glomeratic) if correlated with the Liki-Bembian Group out- cropping to the NW of the basin (e.g. Daly et al. 1992) and may thus represents the basal Precambrian sedimentary unit of the CB. Alternatively, it may also be correlated with the lower Neoproterozoic clastic sequence (BI) of the Mbuji-Mayi basin (Delpomdor et al. 2013a, b; and Chap. 4, this Book) and the Roan Group of the Katangan and Zambian basins.
The base of sequence 0 is not identified in the R05 seismic profile, but suspected in profile L51, at ~ TWT 3,710 ms, which is calibrated using the CDP 850 data at ~9,400 m depth, for a thickness of ~ 1,100 m (Fig. 6.3;
Tables 6.2a, b).
220.127.116.11 Sequence 1
Sequence 1 has a layered seismic pattern characterized by highly continuous and divergent reflectors. In the Mbandaka-1 well its top occurs at ca. 3,960 m, and its bottom at 4,350 m. From this bottom upward to 4,133 m depth, the sequence is represented by a 217 m thick succes- sion of dark-grey calcareous silty shales that grade into argillaceous and dolomitic limestones, with interbedded salt crystals and sparse anhydrite at the base (Esso Zaire 1981a). In the Gilson-1 well, it is represented between 4,503 m depth and the bottom of the well (4,665 m) by alternating beds of sandstone, siltstone, shale and dolomite, with massive dolomite at the base (Esso Zaire 1981b). Sequence 1 is correlated with the Ituri carbonates of the Lindian Supergroup of Verbeek (1970) and represents the M9 sequence of Linol (2013) in the Mbandaka-1 well, and the G10 sequence of Linol (2013) in the Gilson-1 well. It could possibly be correlated also with the carbonate sequence (BII) of the Mbuji-Mayi basin (Delpomdor et al. 2013a, b). Using the CDP 2231 data, its thickness at the position of the Mbandaka-1 well would be ~ 670 m. In profile L51, at CDP 850, Sequence 1 is ~1,220 m thick with its base at a depth of ~ 8,290 m (Fig. 6.3, Table 6.2b).
18.104.22.168 Sequence 2
A second transparent seismic pattern with discontinuous reflectors and one or two strong to medium continuous reflectors in the middle overlies Sequence 1. In the R5 profile (Fig. 6.2a, b; Table 6.2b), it corresponds to an interval Table 6.1 CPD data
(a) For location 2231 along the line R5, close to the Mbandaka-1 well
Times (ms) 1 316 601 691 1,058 1,555 2,041 2,500 5,000
VRMS (m/s) 1,756 1,859 2,085 2,345 3,282 3,865 4,133 4,555 8,000
Depth (m) 1 294 623 786 1,621 2,824 4,023 5,422 15,000
(b) Location 850 along line R51
Times (ms) 0 625 750 1,050 1,450 1,850 2,225 2,975 5,000
VRMS (m/s) 2,000 2,500 2,690 3,145 3,565 4,075 4,480 4,950 6,000
Depth (m) 0 701 999 1,609 2,506 3,615 4,758 7,059 14,422
Table 6.2 Depth for the base of seismic sequences and their thickness, obtained after calibration using the CDP data (a) Location 2231 along the line R5, close to the Mbandaka-1 well
Seismic sequences 5 4 3 2 1 0
Depth (m) 877 2,270 2,844 3,960 4,350a
Thickness (m) 877 1,393 370 1,320 390a
(b) Location 850 along line R51
Seismic sequences 5 4 3 2 1 0
Depth (m) 1,120 2,990 5,410 7,070 8,290 9,400
Thickness (m) 1,120 1,770 2,420 1,660 1,220 1,210